Cathode ray oscillograph sweep circuit



Oct. 27, 1936. 1.. M06. LEEDS CATHODE' RAY OSCILLOGRAPH SWEEP CIRCUIT 2 Sheets-Sheet 1 Filed Sept. 21, 1935 Fig. I.

I I l l I I l l I l I l I l l I I I I l l g I I /a0"+ Fig. s.

Inventor: Laumance, M.Leeds, by Eva/w? Hi Attorney Patented Oct. 27, 1936 UNITED STATES PATENT OFFICE CATHODE RAY OSCILLOGBAPH SWEEP CIRCUIT Laurance McConnell Leeds, Schenectady, N. Y.,

assignor to General Electric Company, a corporation of New York Application September 21, 1935, Serial No. 41,575

Y 7 Claims. (01. 171 95) 7 My invention relates to cathode ray osci1locathode ray screen when the operation is as repgraphs and in particular a recurrent high resented in Fig. 5; and Fig. 7 represents the type frequency time axis generator and circuit conof image that may be obtained, using a sweep nections for such oscillographs. Circuits for this voltage frequency one-eighteenth of the unknown 5 purpose are often called sweep circuits. An imvoltage frequency. 5 portant object of my invention is to provide a Referring to Fig. 1, I have shown at E a sine sweep voltage generator for cathode ray oscillowave curve of an alternating current voltage such graphs capable of producing sweep voltages for as will be used as the sweep circuit voltage source. exceptionally high frequency investigation and It will be noted that those portions of the wave 10 adjustable over a wide range of frequencies which cross the zero voltage line are substantially 10 whereby radio frequency waves up to the order straight. These are the portions lll2 for a 30 million cycles per second may be pictured as rising voltage, and l3-l4 for a falling voltage. a standing wave without distortion. These portions each cover of the time of the In carrying my invention into eiiect, I utilize complete cycle. The portions l|--l2 for the risselected, substantially straight, parallel line volting voltage are parallel to each other as are 15 age varying portions of the waves from a suitable the portions l3--l4 for the falling voltage, and source of alternating current supply for the rethese portions recur in a definite spaced timed curring sweep voltage. A conventional high frerelation to each other. It has been ascertained quency oscillator may be used as the source, and that the lines ll-l2 and l3-l4 are sufiiciently its voltage wave may be altered to segregate straight or linear that the corresponding volt- 20 I the used portions thereof by means of a high ages may be utilized as sweep voltages in a cathvacuum rectifier circuit. The invention also inode ray oscillograph and that the resultant fluocludes a novel synchronous cut oil control of the rescenttrace will be linear to within the thickde y e m wh ch may be used when inness of the trace line. Thus the voltages ll-l2 vestigating unsymmetrical wave shapes to elimmay be used to sweep the cathode ray beam along 25 inate any return trace of the sweep voltage wherea time axis in one direction between definite by unobscured readily interpretable images of the limits in a regularly recurring synchronous relaunknown voltage wave are obtained. tion. It is desirable, however, to eliminate the The features of my invention which are beeffect of the remaining portions of the sine wave lieved to be novel and patentable will be pointed on the tracing movement of the cathode ray if out in the claims appended hereto. For a better clear record are to be obtained.

understanding of my invention, reference is made The manner of eliminating the maximum plus in the following description to the accompanying and minus hump portions of the wave (the perwines wher in Fi 1 is ill tr iv of how an tions l2-l3 and ll-l I) will next be explained,

alternating-current voltage wave is altered, and and for this purpose I prefer to use a high vac- 35 selected straight line portions thereof segregated, uum rectifier circuit shown in Fig. 3, In Fig. 3, according to my invention. l5 represents a source of supply for the sine wave 2 represents those portions of the altered voltage under consideration, namely, the wave E, wave which are utilized as the sweep voltage. Fig. Fig. 1. In Fig. 3, R is a non-inductive resistance 3 represents a preferred form of sweep voltage connected between the source l5 and a full wave 40 atin uit by means of which the fiat rectifier circuit consisting of rectifier tubes l6 topped wave of Fig. 1 may be obtained, Fig. 4 and I1, non-inductive resistances l8 and I9, and Shows the ma of Connecting Such a Sw ep condensers 20 and 2|. The "tubes are reversely v l e genera r to a cathode ray l ph connected so that any current from source I5 and includes the synchronous cut off control for passing through the rectifier circuit flows through 45 the cathode ray beam and the manner of estabtube [6, and resistance l8 in one direction, and lishing a synchronous relation between the sweep through tube l1 and resistance l9 in the opposite v l e and the n n v l nd r inv idirection. such current impulses charge the congation. densers 20 and 2|. Aside from the direction of 5 Fig. 5 represents the timing relation between current flow, the two rectifier circuits are similar, the unknown voltage wave and the sweep voltage that is, resistance I8 is of the same value as wave as utilized in accordance with this invention resistance I 9, the by-pass condensers are similar, when the sweep voltage frequency is one-sixth and the tubes are similar. of the frequency of the unknown voltage; Fig. 6 In the steady state operating condition resistors represents the image that will appear on the l8 and I9 and their associated condensers assume 55 potentials al im polarities as indicated in Fig. 3.

the voltage drop in the tubes ls small income parison to E and E1 and may be neglected. The

constants of the circuit are adjusted so that the tubes pass current only when the voltage E exceeds that corresponding to the straight line portions of the wave' lI-l2 and lc-M of Fig. l.

, Those portions of voltage-E correspondingto points 12-! and l4lsl of Fig. l are then dissipated in resistance R so thatthe humps between the voltageEa equals E minus En remains as. the flat topped curve plotted at the bottomin Fig. 1.

The average or steady state voltage E1 for the positive rectifier branch is shown and the voltages Ea are plotted in the curves of Fig. 1. In practice, instantaneous changes in the slope of the voltage curve Ea will not be obtained due to the inertia of the circuit, but sufflciently rapid change is obtained to make the apparatus-readily usable.

It is alsopossible to so, proportion the resistances R, l8 and I9 that E1 is equal to of E maximum, thus cutting off the straight portions of the voltage wave at points H and l2, l3 and I4 leaving the constant voltage variation portions H-l2 and lS-H each extending in time over a definite predetermined fraction, in this case over exactly V of the complete time cycle of the parent voltage. wave E. I

' The portion of the wave Ea lying between points I |--|2 or between. points lS-ll is used for the practically linear variation of voltage with respect to time; linear to within the thickness of the cathode ray trace line. If the section l|--l2 is used, then section l8-l4 should usually be suppressed or vice versa. This suppression is generally necessary to avoid obscuring of the image trace when unsymmetrical wave shapes are being investigated because the direction of sweep is reversed and in such a case the two images would not be superposed, hence, the effect of one of these voltages should be eliminated except where the unknown voltage wave is symmetrical or it is desirable for any reason to obtain the two images. The constant voltage parts, such as from i2 to I3, of the wave E3 are not bothersome because being constant, they have no sweeping action and would not confuse the image traced.

I accomplish the desired result by cutting oil the beam of the cathode ray ,oscillograph during the return sweep interval so that the sweep voltages which are efl'ective are of. the character pictured in Fig.2.- .Toexplain how this is accomplished, reference is made to the wiringdiagram of Fig. 4, showing my sweep voltage generating apparatus connected'up to a cathode ray :oscillointo three sections for the purpose of shielding, and the dash line inclosures 22, 23 and 24 represent suitable metal shielding inclosures. In the inclosure 22 is a conventional type of variable frequency oscillation generator which corresponds to the source l5 of Fig- 3. It has means such as the variable condenser shown for adjusting its generated frequency. In the inclosure 23 is the high vacuum rectifier circuit, previously ex- -the unknown voltage is of the oscillation generatorto generate tion generator, it has been found possible to plates 26 of the tube 25.

The unknown pulsating voltage to be investigated is designated Ex and is applied to the other or unknown voltage plates 28 of the tube which are, as usual, arranged on a line at right angles to the alignment of the sweep plates across the y axis of the tube, the illustration being diagrammatic; The resistors 21 and 29 are generally known as static drain resistors" and have for their purposes to prevent either of the ungrounded deflection plates 26 and 28 from building up an electrostatic charge which would give a permanent deflection to the cathode beam. These resistors are of the order of ten million ohms so as to have no effect on the impressed voltage to the deflection plates. i

It will be noted that I have provided a wire 45 running from the ungrounded side of the unknown voltage source circuit to the interior of the shield about the oscillation generator l5. When radio frequency, this type of connection is sufllcient to readily enable a frequency which is some definite fraction of the frequency .of the unknown voltage and establish stable synchronization between the two frequencies. In fact, where no shield is provided about the oscillaestablish. stable synchronizing between the two frequencies with no such wire connection but simply by carefully orienting the oscillation generator. This is because this type of generator is very sensitive to electrostatic disturbances and will readily synchronize with such disturbances at some sub-multiple of the disturbance frequency near that for which the oscillation generator is adjusted. It is desirable, however, to shield the apparatus to prevent erratic operation due to various other sources of disturbance,

- but it will be readily seen that there is no difliculty whatever in establishing the necessary synchronous relation between the sweep voltage frequency and that of the unknown voltage by some such connection as is represented by the wire 45. This wire is only one of a variety of expedients that might be used to serve for the purpose of establishing the necessary unknown voltage frequency synchronizing influence on the oscillation generator. 'In' establishing such synchronizing tie. care should be taken to see that none of the unknown voltage Ex gets into the sine wave distortion circuits, as this wou'ldproduce distortion in the image traced. The oscillation generator is, of course, adjustable for a wide variety of frequencies, but its adjustment does not need to be very exact in order to establish synchronism with. the high frequency of the unknown voltage with the oscillation generator operating at some desired sub-multiple of such highfreq'uency. graph. The apparatus shown in Fig. 4 is grouped The tube .25 is cathode 3| provided, as usual, with a with a heating unit 32, an electrode 33 for accelerating the cathode ray beamand a metallic coating 34 about the interior of thejected past cut-off electrode 31 during the I ondary winding 38, usually current pulsation flows jecting of the cathode ray beam. One of the sweep plates 26 and one of the unknown voltage deflecting plates 28 are also connected to the positive side of the cathode voltage potentiometer 36, and this point is grounded to the shield 24 for protection purposes. No attempt has been made to illustrate the details of construction of the tube electrodes as this constitutes no part of the present invention.

1 As thus connected, it will be apparent that the complete fiat topped wave of the voltage E: (Fig. 1) 'will be impressed across the sweep plates 26, and there remains to be explained how the cathode ray is cut off so that the tracing beam indicated at 44 will be swept across the screen in one direction only, thereby obtaining the effect of only that part of the sweep plate voltage depicted in Fig. 2.

For this purpose I have provided a cut off electrode 31 between the cathode 3I and the accelerating electrode 33 which is biased synchronously with the frequency of the sweep voltage E3. The biasing circuitis energized from a secsingle turn winding, associated with the reactance 39 across the output terminals of the oscillation generator I5. It will be apparent then that the voltage impressed on electrode 31 will be in synchronism with the sweep voltage E3. Included in series with the winding 38 there is a high vacuum rectifier tube 40 and a resistance M. The tube 40 allows current to pass only in one direction through this circuit and produces a unidirectional voltage drop, pulsating in character, across resistance 4I. That end of resistance M which is positive in the circuit of rectifier tube 40 is connected to the negative side of the cathode potentiometer resistance 36 and the cut off electrode 31 is connected to the opposite end of resistance 4|. Thus it will be apparent that when a unidirectional in resistance 4|, cut-off electrode 31 will be biased strongly negative with respect to the cathode 3I. While this does not stop the generation of electrons at the heated cathode, it does prevent them from being protime this electrode is sufficiently negative. This negative biasing impulse on electrode 31 should lag about 90 degrees behind the used sweep voltage impulse, and in Fig. 1 .I have represented by the dotted line curves marked E4 the approximate phase relation of the cutoff voltage impulses with respect to the sweep voltage E3 when the useful portion of the sweep voltage is as shown in Fig. 2. I have not found it necessary to use any phase shifting in the cut off voltage circuit to obtain the desired phase-angle relation between voltages E3 and E4 other than the inherent phase shift in the circuit itself which is due primarily to the transformer connection at 38. It will be understood then that this circuit contains phase-shifting means either inherent or otherwise to obtain the proper phase relation of the cut-off biasing voltage E4. The exact point of cut off and reprojecting of the cathode ray beam is not important so long as the beam is stopped during all of the time one of the voltage variations I3--I4 or II-I2 of E3 is impressed across the sweep plates 26 of the tube and is allowed to pass during all of the time the other of these voltages is impressed across the sweep plates so that the beam traces on the fluorescent screen when being deflected along a time axis in one direction only. Thus where the part I I-I2 (Fig. 1) is used as the effective sweep voltage, the beam should be cut ofi between points I2 and I3 and should be reprojected between points I4 and II. I have found that the bias of cut-off grid 31 may be made sufficiently great to cut off the beam very close to point I2 and returned to the screen very close to point, I I.

To coordinate and review the manner of operating a cathode ray oscillograph applying the sweep voltage features of my invention, let us assume we desire to produce a visual picture of the wave shape of a radio frequency current of the character represented at E1: in Fig. 5 having a frequency of million cycles per second, 6 complete cycles being shown. We impress a voltage Ex proportional to such current across the deflecting plates 28 of the cathode ray oscillograph tube. The remainder of the apparatus being in operation, the frequency of oscillation of the oscillation generator I5 is adjusted until the trace on the screen 35 becomes clear and appears as a standing wave such as is shown in Fig. 6. Then we know that the oscillation generator frequency is of the unknown voltage frequency and that the two are in synchronism. It is unnecessary to know what either the frequency of Ex or E3 is to obtain this result. Under these conditions, we may represent the luminous and inactive portions of the sweep voltage cycle by the full and dotted line portions of the E3 curve drawn below curve Ex in Fig. 5. Over the portions I I to 42 and 43 to II, the cathode ray beam isprojected to the screen. Over the portion 42 to 43, the cathode ray beam is cut off. Over the portion II to I2, the sweep voltage increases in a linear manner to trace the section a of Ex (Fig. 5) as Ex (Fig. 6) on the fluorescent screen 35 of the tube 25. Over the portion I2 to 42 the sweep voltage is constant and holds the beam stationary with respect to the time axis while the b wave of Ex (Fig. 5) traces the vertical line B (Fig. 6). Over the time from 42 to 43. the cathode beam is cut off and no part of waves 0, d and e are traced on the screen. If the beam had not been cut 011 over the time from I3 to I4, there would have been traced on the screen another image of the wave dbut in reversed relation as indicated in dotted lines at d (Fig. 5). Whereas here the wave of voltage Ex is unsymmetrical this would, of course, be confusing, and the reason for suppressing the trace over this return of the sweep voltage under such conditions is thus illustrated. Over the period from 43 to II, the sweep voltage is again constant and holds the cathode beam stationary with respect to the time axis while wave 1 (Fig. 5) traces the vertical line F (Fig. 6) on the screen. It will be evident that the vertical line traces B and F (Fig. 6) do not obscure the trace of the voltage wave Ex but serve to more clearly define its length along the time axis.

This operation is repeated at the frequency of E3 so that every sixth wave of Ex is properly traced along the same time axis on the screen, and the persistence of vision allows the trace to be observed as a standing wave. The wave EX (Fig. 5) was assumed to have a frequency of 10 million cycles per second. Let us assume now that we desire to picture a wave EX of 30 million cycles per second. Preferably we would now increase the frequency of the oscillation generator to 5 miiion cycles per second and obtain the same relations as exemplified in Fig. 5. This, however, is unnecesary as we may allow the oscillation generator to oscillate at the same frequency as before, namely, at of 5 million cycles per second, as it will oscillate synchronously with the frequency of Ex at 1/18 of such frequency.

In this case we would obtain a three wave trace of Ex on the screen as represented in Fig. "I over the active V8 cycle of the sweep frequency. The image could, of course, be enlarged by increasingthe plate voltages so that only approximately the center wave of'this group would appear on the screen. Likewise with a frequency for E3 of the frequency of Ex we would obtain a two wave group image of Ex on the screen. The synchronous relation between the sweep voltage and unknown voltage frequencies should be such that one or more complete cycles of the unknown voltage shall occur during the predetermined fraction of the time of the sweep voltage cycle when the sweep voltage is varying in a linear manner. v

It may be thought that an arrangement such as this, wherein the maximum average time of exposureis at the most only V of the total'time, would have poor luminosity. Such is not the case if suitable apparatus is used. Best results may be expected from this apparatus when adequately and effectively shielded to prevent influences from stray fields. It is advisable to ground one terminal of the unknown voltage connection to prevent any capacity effect between this circuit and ground that might alter the characteristics of the unknown voltage wave. In general, the precautions of this character that should be taken are those which should be taken with any high frequency measuring apparatus if reliable results are expected.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I, there fore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention. What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination with a cathode ray oscillograph having the usual sweep and unknown voltage plates, a sweep voltage circuit therefor comprising a source of variable frequency alternat-' ing current having substantially a sine wave voltage, a circuit upon which said voltage is impressed for suppressing the maximum plus and minus portions of such voltage wave and producing a flat-topped trapezoidal shaped voltage wave therefrom leaving the portions of the sine wave between the flattened portions undistorted and each extending over one-sixth of the time of the complete wave, connections for I impressing such fiat-topped voltage wave on the sweep plates of the oscillograph, connections for impressing another pulsating voltage to be investigated across the unknown voltage plates of the oscillograph, and means for establishing a synchronous relation between the sweep voltage frequency and the frequency of said other pulsating voltage such that one or more complete cycles of such other pulsating voltage will occur during one-sixth of the time of the complete sweep voltage cycle.

2. In combination with a cathode ray oscillograph having sweep plates of means for producing a sweep voltage therefor comprising a variable frequency oscillation generator for producing a substantially sine wave voltage, a circuit connecting said source across the sweep plates, said circuit containing a resistance, a

tions, and means for pair of rectifiers reversely connected across said circuit beyond the resistance so as to produce a voltage drop in said resistance when the rectifiers pass current in either direction, each rectifier having a resistance shunted by a condenser connected in series therewith across said circuit, the constants of the rectifier load circuit thus formed being suchas to change the sine wave voltage produced by said generator into a symmetrical flat-topped wave form which is applied to the sweep plates of theosciliograph.

3. A circuit for obtaining a trapezoidal shaped alternating voltage wave suitable for the sweep voltage of a cathode ray oscillograph comprising a substantially sine wave voltage source of alternating current, a resistance in series with said source, and a rectifier circuit connected across said source through said resistance, said rectifier circuit having parallel branches each containing a rectifier in series with parallel connected resistance and capacity, the parallel branches being similar except that the rectifiers are reversely connected therein, the constants of said sweep voltage circuit beingsuch with respect to the voltage of the source that the rectifiers pass current only when the voltage of said source reaches one-half its maximum value in either direction and the voltage components of said source in excess of one-half of such maximum value in either direction are dissipated as voltage drop in the first mentioned resistance.

4. In combination with a cathode ray oscillograph having a tracing screen and cathode ray beam producing, projecting, and deflecting electrodes,a sweep voltage circuit therefor comprising means for producing a variable frequency alternating sweep voltage having similar, regularly recurring, substantially linear, voltage increasing portions and similar, regularly recurring, substantially linear, voltage decreasing portions, the voltage-increasing and voltage-decreasing portions alternating in time and being separated by constant voltage portions, connections for impressing said voltage across a pair of deflecting electrodes of said oscillograph, electrode means for said oscillograph which, when biased by a suitable voltage, prevents the cathode ray beam from tracing on the screen of said oscillograph, and a circuit energized synchronously with the frequency of the sweep voltage circuit for synchronously biasing the last mentioned electrode to prevent the cathode ray beam from tracing on the screen during alternate linear voltage varying portions of said sweep voltage.

5. In combination with a cathode ray oscillograph having a screen, a cathode beam producing electrode, a cathode beam projecting electrode and sweep plates for sweeping ,the beam across the screen as it is projcted thereon, a source of I alternating voltage connected across said sweep plates having characteristics which would ordinarily sweep the cathode ray beam across said screen alternately in opposite direcpreventing the tracing action of the beam on the screen in one of said directions comprising a cut-off electrode for said oscillographlocated between the cathode beam producing electrode and the cathode beam prorelation with respect to the sweep voltage as to prevent projection of the cathode ray beam on the screen during the period when it would otherwise be swept in one direction only.

6. In combination, a cathode ray osclllograph having a screen, a cathode beam producing electrode, a cathode beam projecting electrode and sweep plates for sweeping the projected beam across the screen, a source of alternating voltage connected across the sweep plates of such character as would ordinarily sweep the beam across the screen alternately in opposite directions, means, for preventing the tracing action of said beam in one of said directions comprising a cutoff electrode located adjacent the projecting elec:

trade of the oscillograph, and a circuit for intermittently impressing a voltage on the cut-off electrode sufiicient to neutralize the projecting eifect of the projecting electrode, said circuit containing a resistance and a rectifier and being energized from said source of alternating voltage, the end of said resistance which is negative in said rectifier circuit being connected to the cut-oil electrode and the other end being connected to the cathode beam producing electrode of the oscillograph, said circuit including phase-shifting 7 means whereby the phase relation between the graph having a tracing screen, electrodes for producing and projecting a cathode ray beam on said screen and two sets of plates for respec- ,being separated in time by constant voltage portions, a source of pulsating voltage to be investigated connected to the remaining plates of sai oscillograph, means including means for adjus ing the frequency of the sweep voltage for establishing a synchronous relation between the frequencies of the sweep voltage and the voltage to be investigated such that at least one complete wave of the last mentioned voltage will occur duringasingle voltage-varying period of the sweep voltage, and a cathode ray beam cut-off electrode for said oscillograph energized synchronously from the sweep voltage circuit for intermittently V 

